Most commercially-available pan/tilt mechanisms are motor-on-motor devices. One rotary motor, such as a brushless DC motor or a stepper motor, is used to move an object about one axis, while a second motor controls motion about another axis. Because the second motor must drive the first along with the object, this type of design is inherently inefficient. While these devices may be powerful and reliable, they tend to be large, heavy, slow, and to require high power.
One type of device is representatively shown in U.S. Pat. No. 5,204,573 "Benderson et al."). This reference appears to disclose a gimbaled magnet surrounded by three orthogonally-positioned coils, with no magnetic pole structure. This patent apparently teaches that the coil whose axis is aligned with the magnet axis cannot produce torque, so effectively only two coils are needed. This patent shows two forms, a moving-magnet fixed-coil form and moving-coils fixed single-axis magnet form. This device orients a magnet with respect to a set of coils, but the coils completely surround the magnet and must be pulled apart to create an opening. Aside from being inconvenient, this distorts the magnetic field which controls the magnet. In fact, it is necessary to calibrate this motor using a procedure specifically designed for the device. The control strategy is open-loop, and so has limited accuracy, especially in applications involving a changing environment.
U.S. Pat. No. 4,785,212 ("Downer et al.") discloses an annular magnet (or solenoid coil) carried in a spherical housing by magnetic suspension bearings, and acted upon by three opposed pairs of coils on the surface of the housing to control the angular position of the magnet axis in two directions. The positioning of the coils on the surface of the sphere is not defined, but they are shown diagrammatically as being on three orthogonal axes.
U.S. Pat. No. 4,600,871 ("Idoguki et al.") appears to disclosure a magnet in a spherical bearing with its axis perpendicular to two radial flux fields in a plane established by paired polepieces at right angles to one another and pairs of coils operatively arranged to produce flux.
U.S. Pat. No. 5,892,310 ("Kempas") appears to disclose a hemispherical arrangement of a shell carrying segmented coil pairs in a fixed permanent magnet field.